The nurses' union wants minimum nurse to patient ratios to be legally required in hospitals.
More than 150 frontline staff at Auckland Hospital's emergency department have signed a complaint to management saying chronic staff shortages expose them to unacceptable risk.
The complaint has been signed by doctors, nurses and healthcare assistants and lodged under the Health and Safety at Work Act.
It warned hospital bosses "safe staffing levels" were "consistently breached".
The staff said the maximum ratio of nurses to patients should be one to four, but patient numbers were often higher, putting staff and patients at risk.
Nurses Organisation delegate Nico Woodward told RNZ's Nine to Noon the emergency department was full to overflowing around the clock for weeks at a time.
"We're taking on loads that are becoming unsafe," Woodward said.
Low numbers of nurses to patients was leading to longer wait times and worse outcomes for patients, he said.
Staff needed breaks to recharge, but often shortages meant there was not the necessary cover, Woodward said.
"We work in a very challenging environment."
Although Te Whatu Ora/ Health New Zealand had agreed in principle to safe staffing levels, it continually breached them, he said.
Te Whata Ora group director of operations Mike Shepherd said the emergency department (ED) and clinical decision unit (CDU) had faced high demand, with pressure on staff numbers recently.
However, people who needed urgent hospital care would receive it, Shepherd said.
"We acknowledge that at times staffing in our ED/CDU has been very challenging, especially over the past six months.
"A number of factors have impacted on the daily running of the unit, and these include high demand, staff vacancies, staff turnover and sick leave.
"We have been providing staff from across the hospital to support, and have either implemented or are working towards implementing a range of short and longer term solutions to help alleviate the challenges," he said.
Te Whatu Ora had filled all staff vacancies in the two departments and had increased the number of nurses rostered on in areas of pressure, Shepherd said.
They would soon introduce an acuity system to help ensure appropriate staffing levels, he said.
"We're committed to working with our kaimahi (staff) to put in place further solutions to the challenges they're facing," Shepherd said.
Nurses Organisation president Anne Daniels said getting legislation on nurse to patient ratios was a top priority for the union.
"Successive governments and nurse employers have completely and totally let us down by not actually engaging in this space," Daniels said.
Attempts had been made to include a safe staffing level clause in the nurses collective agreement, but Te Whatu Ora had "blatantly refused to even start the discussion", she said.
The majority of frontline staff had signed the complaint, Woodward said.